Theory of Natural Selection dwells on :

The correct option is B Natural selection acting on favourable variations
According to Darwin's theory of evolution, more individuals are produced in each generation that can survive. Phenotypic variation exists among individuals and the variation is heritable. Only the organisms best adapted to their environment tend to survive and transmit their genetic characters in increasing numbers to succeeding generations while those less adapted tend to be eliminated.

So, the correct answer is 'Natural selection acting on favourable variations'